The purpose of this study is to investigate the intrinsic material characteristics of plasma membranes and to relate the material properties to molecular structure in the plane of the membrane. Study of the material behaviour of blood and endothelial cell membranes provides the diversity necessary to investigate membrane commonality, membrane differentiation, and integration of cells with tissue. Prescription of the membrane "shear" modulus will be obtained as a function of time, temperature, and strain level. Relating this to molecular structure through statistical thermodynamics provides information relative to molecular network cross-linking and entanglement. Important aspects of the study include: (1) determination of solid/ semisolid/liquid behaviour in normal red cell membranes; (2) determination of the material nature of membrane differentiation and maturation in the erythroid series; (3) determination of fatigue mechanisms in the red cell membrane resulting from exposure to high shear conditions; (4) determination of red cell fragmentation mechanisms by studying plastic (semi-solid) behaviour of the membrane; (5) determination of the role of the endothelial cell in capillary rigidity; (6) extension to white cell studies.